(1) The fundamental principle of using friction is to prevent the threaded components from rotating relative to each other. Common methods include the use of elastic washers such as flat spring washers or corrugated spring washers, double nuts, and nylon locking rings. These methods rely on increasing the friction between the mating surfaces to maintain the tightness of the connection.
(2) Another approach involves breaking the engagement between the thread pair by converting it into a non-moving joint, which eliminates the chance of rotation. This can be achieved through methods like welding, riveting, or using metal bonding agents. For instance, thread-locking compounds act as a form of metal adhesion. Once heated to a specific temperature, these sealants lose their effectiveness and can be removed multiple times without damaging the threads.
Improving the fatigue strength of bolted joints is crucial, as most failures in tensioned bolts are due to fatigue. To enhance this, the focus should be on reducing the stress on the bolt and improving its resistance to cyclic loading. Key strategies include:
(1) Applying the correct pre-tension force during installation. When the rigidity of the joint remains constant, increasing the pre-tension force can significantly improve the bolt's fatigue resistance while preventing unwanted gaps in the assembly.
(2) Enhancing the load distribution along the threads. In typical applications, the load is not evenly distributed among all the threads. The first few threads near the nut bear the majority of the load, while the later threads experience much less stress. To address this, various techniques such as using a slotted nut, an internal tapered nut, or a ring groove nut can be employed. Additionally, using a wire thread insert is also an effective solution.
To minimize additional stresses and stress concentrations, it's important to ensure that the bolt axis remains perpendicular to the contact surface of the connected parts. This helps the bolt only withstand tensile forces rather than bending moments. Practical solutions include adding a spherical washer, using a wedge washer that matches the inclined surface of the component, or incorporating features like bosses or recessed seats.
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